The invention is broadly concerned with improvements in the field of controlling gaseous contaminants from combustion and in particular from the exhaust of internal combustion engines. In particular, polyfunctional catalysts having the ability to convert unburned hydrocarbons, carbon monoxide, and nitrogen oxides to less harmful forms, and a method of operating such catalysts are disclosed.
The problem of substantially converting gaseous contaminants or pollutants in automotive exhaust emissions to less harmful forms has been the subject of much research activity, especially in recent years. There are four principal gaseous components of auto exhaust which are of current interest, namely, unburned hydrocarbons, carbon monoxide, nitrogen oxides, and sulfur oxides. Standards for all these components, except sulfur oxides, have been set by the U.S. Government and which new automobiles already are required to meet. Until recently, adjustments to engine operating conditions have been sufficient to meet the standards. As these standards become increasingly severe, it has been necessary to introduce new methods for removing these contaminants. Most recently, catalysts have been used to oxidize the unburned hydrocarbons and carbon monoxide. In the near future catalysts may be needed to meet more stringent limitations on the nitrogen oxides and sulfur oxides (measured as sulfates and expressed as sulfuric acid) contained in exhaust gases. Removal of nitrogen oxides is accomplished by reducing the oxides to molecular nitrogen. Although sulfur dioxide is produced by the engine combustion process, if the sulfur dioxide is not oxidized to sulfur trioxide or sulfuric acid, then no sulfates are measured. In conventional exhaust catalysts which usually operate under oxidizing conditions, nitrogen oxides are not significantly reduced but sulfur dioxide is oxidized and sulfur trioxide is produced. Improved catalysts and/or revised operating conditions are required to remove all three of the principal contaminants simultaneously without oxidizing the sulfur dioxide present in exhaust gases.
U.S. Pat. No. 3,331,787 discloses a typical precious metal catalyst (platinum and palladium are preferred) which can be used for oxidation of hydrocarbons and carbon monoxide emitted in auto exhaust. Such catalysts are operated with an excess of oxygen present to facilitate the oxidation process. Even should the free oxygen be limited, at least some of the sulfur dioxide present is converted to sulfur trioxide or sulfuric acid. Since removing nitrogen oxides involves the reduction of nitrogen oxides to molecular nitrogen, such reduction is not favored by the conditions which are used to oxidize the hydrocarbons and carbon monoxide to water and carbon dioxide. The two reactions normally require different conditions. For oxidation, an excess of oxygen should be present (fuel-lean), while if nitrogen oxides are to be reduced, it is generally necessary to operate with a deficiency of oxygen (fuel-rich).
Various proposals, as represented by U.S. Pat. Nos. 3,565,574 and 3,741,725, have been made to use two or more catalyst beds in sequence to treat exhaust gases (both catalysts being nickel-based in 3,565,574, and platinum-palladium-metal oxide being used in sequence in 3,741,725). Usually nitrogen oxides are reacted first with the exhaust gas being maintained fuel-rich, followed by injection of air to create a fuel-lean condition suitable for oxidizing of hydrocarbons and carbon monoxide.
A precious metal catalyst (platinum and rhodium) used solely for control of nitrogen oxides is disclosed in U.S. Pat. No. 3,806,582. This catalyst is operated in the presence of an added reducing gas. It is intended that this catalyst be applied to nitric acid plant tail gas, where addition of a reducing gas is feasible. This is less practical during operation of automobiles and a fuel-rich operation would be used instead. However, operating an automobile with an excess of fuel is less economical and produces substantial amounts of unburned hydrocarbons and carbon monoxide, which must be removed by an oxidation catalyst.
In U.S. Pat. No. 3,840,471 a catalyst comprising platinum and rhodium alloyed or mixed with a relatively smaller amount of base metal (nickel in the example given) on an inert support ("Torvex" by E. I. duPont de Nemours & Co. in the example given) is disclosed which will (i) oxidize hydrocarbons and carbon monoxide, or (ii) reduce nitrogen oxides with the addition of a reducing fuel. However, the patent does not disclose the simultaneous removal of all three of these contaminants and suggests that if the catalyst is used to remove all components that sequential operation such as discussed above would be required. Conditions would be adjusted by adding a reducing gas for removal of nitrogen oxide, or alternatively, by adding air for oxidation of hydrocarbons and carbon dioxide.
A previously proposed polyfunctional catalyst, disclosed in U.S. Pat. No. 3,370,914, is capable of removing all three major contaminants simultaneously. A reduced nickel or alumina catalyst promoted by alkali and alkaline earth metals is used to cause the exhaust gases to come to chemical equilibrium. It is shown in the patent that if thermodynamic equilibrium could be achieved, the contaminants would be substantially removed. Such a catalyst will promote the equilibrium of the proposed reactions, but it is believed that this catalyst does not retain this activity for a commercially practical period of time and the amount of catalyst required for automotive use could be excessive for satisfactory emission control.
In summary, it is known in the prior art (1) to use precious metal catalysts for oxidizing carbon monoxide and hydrocarbons (U.S. 3,331,787), (2) to use precious metal catalysts for reducing nitrogen oxide in the presence, of reducing gas (U.S. 3,806,582), (3) to use an alloy or mixture of precious metals and a relatively smaller amount of base metals in a catalyst which can be used for either oxidation or reduction when suitable operating conditions are provided (U.S. Pat. No. 3,840,471), (4) to adjust exhaust gas compositions so that either the oxidation or the reduction is achieved (U.S. Pat. No. 3,565,574 and 3,741,725), and (5) to use a base metal catalyst to promote equilibrium of reactions favorable for removing each of the three principal contaminants in exhaust gases (U.S. Pat. No. 3,370,974).
Another catalyst is disclosed in U.S. Pat. No. 3,883,444, having the capability of reacting all three of the major contaminants simultaneously when stoichiometric amounts of oxygen are present in the exhaust gases. At low space velocities this simultaneous conversion was fairly complete for a short period of time, but similar results were not obtained at higher space velocities. Palladium alone is used in the catalysts in combination with large amounts of cobalt and nickel oxides. However, palladium is sensitive to the sulfur and lead contents of the fuel, and has little ability to retain its activity when operated with an engine operated at essentially stoichiometric conditions, and the use of palladium as the only platinum group metal in the catalyst may not be suitable for use where high levels of conversion of contaminants must be met for extended periods of time.
What has been needed, but not shown in the foregoing prior art, is a catalyst having the ability for a commercially acceptable period to oxidize hydrocarbons and carbon monoxide, without producing significant amounts of sulfur trioxide or sulfuric acid in the exhaust gases or in the atmosphere into which the gases are discharged, while at the same time and with the essentially same operating conditions, to reduce nitrogen oxides without producing significant amounts of hydrogen sulfide, thus avoiding adjusting catalyst operating conditions to produce separate oxidizing and reducing zones. Such a result has been acomplished in the present invention by a novel catalyst which is used in conjunction with an internal combustion engine in which the air-fuel ratio is closely controlled. The catalyst of this invention may also be used in plural catalyst operations, for instance, in conjunction with an oxidation catalyst in separate reaction zones.